Hydraulic fracturing can impact water quality through several distinct pathways: chemical spills on the surface, failures in the steel and cement casings that line wells, contamination from wastewater disposal, and methane migration into nearby drinking water wells. The EPA has confirmed that fracking activities can affect drinking water resources under certain circumstances, though data gaps have made it difficult to estimate how often this happens nationwide.
What Goes Into a Fracking Well
A single hydraulically fractured well requires between 1.5 million and 16 million gallons of water, depending on the rock formation, well orientation, and number of fracturing stages. That water is mixed with sand (which props open fractures in the rock) and a cocktail of chemical additives, each serving a specific function underground.
The chemical mixture includes acids to dissolve minerals near the wellbore, biocides to kill bacteria that could clog the well, friction reducers to help fluid flow more easily, and scale inhibitors to prevent mineral buildup. Some of these additives contain compounds like methanol, toluene, and ethylene glycol. Others include gelling agents to thicken the fluid and “breakers” that thin it back out once the fracturing is complete. The exact recipe varies by operator and geology, but the fluid that comes back to the surface after fracturing picks up additional contaminants from deep rock formations, making it significantly more hazardous than what went in.
Well Casing Failures
The primary barrier between fracking fluids and groundwater is the well casing: layers of steel pipe cemented into the surrounding rock. When that barrier fails, gases and liquids can leak out through cracked casing or travel upward through gaps in faulty cement between the pipe and the rock wall.
Failure rates vary widely depending on the region and time period studied. An analysis of over 41,000 oil and gas wells in Pennsylvania found that 1.9% showed a loss of structural integrity overall, but unconventional shale gas wells were six times more likely to have problems than conventional wells drilled during the same period: 6.2% versus 1.0%. Other studies have found rates ranging from 2.6% to 6.3% for wells drilled between 2005 and 2013. Offshore data from the Gulf of Mexico shows 11 to 12% of wells developed sustained pressure in their outer casings, a sign of compromised integrity.
Defective, insufficient, or improperly installed cement and casing are the most common violations. In Pennsylvania alone, state regulators have confirmed more than 100 cases of water well contamination from oil and gas activities since 2005. In 24 of those cases, the state Department of Environmental Protection concluded there had been a failure to prevent migration of contaminants into fresh groundwater.
Methane in Drinking Water Wells
A study of 68 private drinking water wells in northeastern Pennsylvania and New York found that methane contamination rose sharply the closer a well was to an active drilling site. Average methane concentrations in groundwater near active drilling areas fell within the federal action level for hazard mitigation (above 10 milligrams per liter), and the highest reading recorded was 64 milligrams per liter, far beyond the 28 milligram threshold that triggers immediate action.
The type of methane matters too. Wells closer to drilling sites were more likely to contain thermogenic methane, the kind that originates from deep geological formations rather than from shallow biological processes like decomposing organic matter. This distinction is important because thermogenic methane in shallow groundwater suggests a migration pathway from deep drilling zones to the water table, likely through compromised well casings or natural fracture networks.
At high concentrations, dissolved methane can cause sputtering faucets, bubbling noises in your well, and visible white gas bubbles in water. In enclosed spaces, it creates an explosion risk.
What Comes Back Up: Flowback and Produced Water
After a well is fractured, a portion of the injected fluid returns to the surface as “flowback water,” followed by “produced water” that continues flowing for the life of the well. This wastewater has been in contact with rock formations that are millions of years old, and it picks up naturally occurring radioactive materials, extremely high salt concentrations, and other contaminants along the way.
Flowback water from the Marcellus Shale formation (which underlies parts of New York, Pennsylvania, West Virginia, and Ohio) contains radium levels far exceeding anything found in natural surface water. Combined radium-226 and radium-228 levels as high as 6,540 picocuries per liter have been reported, compared to the EPA’s drinking water standard of 5 picocuries per liter for combined radium. The salt content is equally extreme. Chloride concentrations in produced water from Pennsylvania oil wells range from 5,800 to 68,000 milligrams per liter. For context, the EPA’s secondary drinking water standard for chloride is 250 milligrams per liter.
When this wastewater is discharged through treatment plants that aren’t equipped to handle it, those contaminants reach rivers and streams. One USGS study measured bromide at 75 milligrams per liter and chloride at 8,200 milligrams per liter in the outfall of a centralized waste treatment facility handling oil and gas wastewater in Pennsylvania. Upstream of the facility, bromide was below detection limits and chloride was just 14 milligrams per liter. Bromide is a particular concern because it reacts with chlorine used in downstream drinking water treatment to form disinfection byproducts linked to cancer risk.
Surface Spills
Not all contamination comes from underground. Spills of fracking fluids, chemicals, and produced water at the surface are a routine occurrence. In California alone, 1,029 produced water spill incidents were reported between 2006 and 2020. The EPA identified large-volume spills and spills with high chemical concentrations that reach groundwater as one of the scenarios most likely to cause severe impacts to drinking water.
Storage is another vulnerability. Wastewater held in unlined pits can seep into underlying groundwater. The EPA specifically flagged this as a high-risk scenario for contamination.
Orphan and Abandoned Wells
Even after a fracking well stops producing, it can continue to threaten water quality for decades. Hundreds of thousands of unplugged wells across the United States are classified as “orphans,” meaning no company or individual is legally responsible for sealing them. These wells act as open conduits, allowing hydrocarbons and brines to migrate from deep formations into water supply aquifers.
A USGS geospatial analysis identified 117,672 documented orphan wells nationwide. Of those, 64,203 sit within principal aquifer systems that supply drinking water to large populations. The risk is compounded by factors like well age (older wells have weaker casings), groundwater withdrawal rates in the surrounding area, and the number and density of orphan wells in a given region. As cement and steel degrade over time, even wells that were properly cased when drilled can eventually lose their integrity.
What to Test for if You Live Near Drilling
If your home relies on a private well in an area with oil or gas activity, baseline water testing before drilling begins is critical for establishing what your water quality looked like beforehand. Penn State Extension recommends testing for a specific set of parameters tied to drilling contamination, including methane and ethane, barium, strontium, bromide, chloride, sodium, and BTEX compounds (benzene, toluene, ethylbenzene, and xylene). Gross alpha radiation testing can detect radium and other naturally occurring radioactive materials that may migrate from deep formations.
More routine parameters like pH, iron, manganese, and arsenic are also worth tracking because drilling disturbances can mobilize naturally occurring metals in surrounding rock. Methane is especially important to monitor over time. If your water develops a bubbling sound, your faucets start sputtering, or you see white gas bubbles, those are signs of elevated dissolved methane that warrant immediate testing.